Known in the art is a process (cf. USSR Inventor's Certificate No. 311887 Cl. C 07 C 31/20, 1971) for producing ethylene alcohol namely 2-butenediol-1,4 by hydrogenation of 2-butynediol-1,4 with hydrogen in an organic solvent medium in the presence of oxalic acid on a catalyst of a mixture of palladium black and black of a metal of the platinum group (except platinum per se). In this process, wherein use is made of palladium black and ruthenium black, the process selectivity at a temperature of 20.degree. C. is equal, relative to 2-butenediol-1,4, to 96% at full conversion; stereospecificity relative to cis-2-butenediol-1,4 is equal to 97%.
This prior art process has disadvantages residing in the following:
(1) the necessity of using a pure starting alcohol and pure hydrogen;
(2) hydrogenation is conducted in an organic solvent medium in the presence of oxalic acid, thus adding to the production costs;
(3) mechanical strength of the catalyst in the form of a metal black is insufficient, wherefore separation of the catalyst from the reaction mixture is hindered;
(4) difficulties are encountered in regeneration of the catalyst after lowering of its activity: thermal treatment cannot be applied due to sintering of the black particles, wherefore in this case regeneration of the catalyst should be preferably effected by its dissolution to recover noble metals and subsequently prepare a fresh portion of the black. However, this regeneration scheme is laborious.
In another prior art process (cf. FRG Pat. No. 2,431,929 Cl. C 07 C 33/02, 1976; U.S. Pat. No. 4,001,344 Cl. 260-635, 1977; British Pat. No. 1,504,187 Cl. B1E, 1978) 2-butenediol-1,4 is produced by hydrogenation, with hydrogen, of a technical product, namely a 30% aqueous solution of 2-butynediol-1,4 on a catalyst containing 0.05 to 2% by mass of palladium, 0.05 to 1% by mass of zinc and cadmium supported by .gamma.-alumina at a temperature within the range of from 60.degree. to 75.degree. C. under a hydrogen pressure of from 1 to 16 atm. The yield of 2-butene-1,4-diol is 88 to 92.5% of the theoretical. The above-indicated starting technical product is prepared by a conventional Reppe process using acetylene and an aqueous solution of formaldehyde in the presence of copper acetylenide.
A disadvantage of this prior art process for producing 2-butenediol-1,4 resides in the formation of resinous products in an amount of from 7.5 to 12% by mass. Furthermore, other disadvantages are associated with a relatively low yield of the desired product, the necessity of its separation from the catalyst by filtration and accompanying losses of the noble metal (palladium), difficulties of regeneration of the catalyst after lowering of its activity (the regeneration comprises the recovery of the noble metal from the spent catalyst and the preparation of a fresh portion of the catalyst). The obligatory stage of separation of the desired product from the catalyst and considerable labor-consumption in the regeneration stage complicate the overall process.
In the process disclosed in British Pat. No. 888,999 Cl I/I/A31B1, 1962, acetylene alcohols in the form of commercial products, e.g. commercial 2,6-dimethylocten-2-in-7-ol-6 (dehydrolinalool) and commercial 2-methylbutyn-3-ol-2 (dimethylethynylcarbinol) are selectively hydrogenated with hydrogen into corresponding ethylene alcohols under atmospheric pressure of hydrogen in a medium of a solvent or without on a catalyst containing up to 20% by mass of palladium and 0.1 to 20% by mass of lead deposited on barium sulphate, calcium carbonate or barium carbonate, activated coal or magnesia at a temperature within the range of from 10.degree. to 75.degree. C. The hydrogenation process is conducted discontinuously. Separation of the desired product from the catalyst is effected by vacuum distillation. The yield of 2,6-dimethyloctadien-2,7-ol-6 (linalool) is 90.5%, that of 2-methylbuten-3-ol-2 (dimethylvinylcarbinol) is 97% as calculated for the starting acetylene alcohol.
The discontinuous scheme of the hydrogenation process and the use of vacuum distillation substantially increase the process duration. The vacuum distillation also necessitates the use of expensive process equipment and lowers the yield of the desired products.
It should be noted that the liquid-phase hydrogenation of acetylene alcohols under continuous flow-through conditions on the catalysts employed in all the above-described prior art processes is impossible due to unavoidable high losses of noble metals of the catalyst. Furthermore, a common disadvantage of the above-mentioned prior art processes is the difficult regeneration of catalysts on carriers and catalysts in the form of a noble metal black. Such regeneration contemplates dissolution of noble metals in a mixture of nitric acid, recovery of chlorides of noble metals and preparation of a fresh portion of the catalyst. All these operations take a long time and necessitate the use of special sophisticated process equipment.